1. Field of the Invention
The present invention relates primarily to digital magnetic recording systems. The invention may be considered as a system that converts binary data into ternary data and records the ternary data on a magnetic medium. The ternary data read from the medium is converted back into the corresponding binary format. The concepts of the present invention may also be utilized in digital communication systems.
2. Description of the Prior Art
Present day digital magnetic recording systems typically record binary data by utilizing binary valued write current waveforms. For example, in the conventional return-to-zero (RZ) and non-to-return-to-zero (NRZ) formats each cell of the medium is magnetized either in the positive direction or the negative direction to represent the two binary data states. In a modified NRZ format (NRZI) a transition from the existing polarity of magnetization to the opposite polarity of magnetization is recorded for a binary ONE and no transition is recorded for a binary ZERO.
It is a desideratum of the magnetic recording art to increase the information density stored on the medium by, inter alia, packing the magnetic flux transitions as closely as possible on the medium. Depending on the configuration of the magnetic interface, non-linear distortions and intersymbol interference degrade data recovery reliability because of such factors as pattern dependent amplitude attenuation, which effects bit resolution, and timing displacement anomolies such as peak shift. Conventionally complex and hence expensive signal processing channels are provided to enhance data recovery reliability in such systems utilizing high bit packing density of the medium.
Another technique utilized to increase the storage efficiency of the medium is to encode the binary data by various run-length-limited codes. Although providing significant improvements in recording density efficiency, systems utilizing such codes are nevertheless restricted by the basic limitations of the interface.
It should be noted that only the position of the transition, but the polarity of the transition, has been commonly used to convey information in the present day saturated digital magnetic recording systems.
Another basic requirement in digital magnetic recording systems is overwrite of old data. When new data is recorded in prior art saturated magnetic recording systems, saturation current must be utilized to effectively erase the old data. In accordance with the parameters of the magnetic interface, the saturation current required may be excessive for undistorted recording of the new data. This results in an inhomogeneously erased medium on which the new data is recorded resulting in degraded surface noise and SNR. In the present invention the high frequency erase signal effectively erases the medium without the requirement of excessive current which in the prior art caused the degradation in data.
Additionally, in conventional prior art saturated recording utilizing, for example, run length limited codes, the frequency spectrum of the recorded information contains both low and high frequency components. As is known, the low frequency components require high overwrite current which has a tendency to distort the new data. In the present invention the write information contains no low frequency components. Therefore, the CRA waveform comprising high frequency dither and CRA breaks, which are themselves high frequency waveforms, can effectively erase the old data, without excessive current which in prior systems distorted the data.
It is generally known that the information density of magnetic recording systems may be enhanced by providing higher order recording than binary. For example, the NRZ and RZ formats may be enhanced by utilizing the zero write current state as an information bearing condition in conjunction with positive and negative magnetization. Such a ternary system will not provide the necessary function of overwrite of old data when new data is recorded. A separate erase cycle may be utilized to obviate the problem but this is generally considered unacceptable in high speed present day digital magnetic recording systems.